1. Field of the Invention
The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with a boiling water reactor (BWR) fuel assembly having exterior springs for axially positioning fuel rod spacers so that locating tabs on spacer capture rods within the fuel rod bundle which degrade critical heat flux (CHF) performance can be eliminated.
2. Description of the Prior Art
Typically, large amounts of energy are released through nuclear fission in a nuclear reactor with the energy being dissipated as heat in the elongated fuel elements or rods of the reactor. The heat is commonly removed by passing a coolant in heat exchange relation to the fuel rods so that the heat can be extracted from the coolant to perform useful work.
In a typical boiling water reactor (BWR) fuel assembly, a bundle of fuel rods are subdivided into four separate mini-bundles by a central water cross and each mini-bundle is supported in laterally spaced-apart relation by a plurality of axially displaced grids or spacers, for example six in number. Then, all four mini-bundles of the fuel assembly are encircled by an outer tubular channel having a generally rectangular cross-section. The outer flow channel extends along substantially the entire length of the fuel assembly and interconnects a top nozzle with a bottom nozzle. The bottom nozzle fits into the reactor core support plate and serves as an inlet for coolant flow into the outer channel of the fuel assembly. Coolant enters through the bottom nozzle and thereafter flows along the fuel rods removing energy from their heated surfaces. Such BWR fuel assembly is illustrated and described in U.S. Pat. No. (4,560,532) to Barry et al.
A BWR consists of several hundred fuel bundles, each enclosed in its own outer tubular channel. Also, as mentioned above, each mini-bundle has a plurality of spacers axially spaced apart along its fuel rods. The spacers are needed to maintain the desired fuel mini-bundle configuration and to prevent excessive fuel rod bow and flow induced vibrations, and thereby provide significant benefits from a structural standpoint.
In one arrangement disclosed in the Barry et al patent to maintain the axial positions of the spacers in the BWR fuel assembly, two pairs of tabs are provided below and above each spacer on special fuel rods, called spacer capture rods. The specific purpose of these tabs is to prevent axial dislocation of the spacers. The width of each tab (2-3 mm) is proportional to or larger than the width of dimples provided within the spacer to transversely support or hold the fuel rods. As such, these tabs can be expectd to strip off a similar amount of liquid film cooling the fuel rods, as the spacer dimples which hold the fuel rods.
Stripping off the liquid film causes premature dryout and critical heat flux (CHF). It is well known that such a stripping mechanism can cause significant deterioration in the critical power ratio (CPR). Again this effect causes greatest degradation in CPR margins, if the tabs are located flush against the spacer as the local disturbances to the liquid film on the fuel rods are compounded. Also, depending on the radial variation of rod bundle CHF characteristics, this process could further be enhanced if the tabs are on a rod located in a "limiting" CHF radial location. The CHF phenomenon in BWRs is of paramount importance in characterizing the power rating of the plant. Hence degradation of this margin should be minimized.
Another arrangement for maintaining the desired axial positions of spacers in the BWR fuel assemblies is to employ spacer holder rods which replace and are arranged in parallel with the fuel rods of the fuel assembly. Such approach has been described in U.S. Pat. No. (3,802,995) to Fritz et al and U.S. Pat. No. (4,508,679) Matzner et al. In Fritz et al, a fuel rod in a central fuel rod position is replaced by a spacer capture water tube having a plurality of radially extending, axially spaced fins or lugs, each of which is adapted to engage a respective one of the spacers to retain it in fixed axial position. Matzner et al uses lugs on moderator tubes as in the arrangement of Fritz et al for maintaining the desired axial positions of spacers of a different construction along the fuel rods. The fritz et al arrangement is applied to retain the spacer construction shown in Lass et al (U.S. Pat. No. 3,654,077). The Lass et al spacer has a peripheral support band and a plurality of sheet metal divider members and divider wires. In contrast, the Matzner et al spacer has a plurality of ferrules welded together and encompassed by a peripheral band. The band has a pair of inwardly curved projections near each corner to serve as lead-ins when the outer tubular flow channel is inserted over the fuel assembly. The peripheral band also has a pair of outwardly extending lobes near each corner for providing predetermined spacing between the band and the surrounding coolant flow channel. However, a particular drawback of these spacer holder water tubes is that they take up space in the fuel assembly which could otherwise be used to receive fuel rods, thus reducing the operating economy of the reactor.
Yet another arrangement is to dispose the spacer in interlocking relation to the outer tubular channel of the fuel assembly. Such arrangement is disclosed in a U.S. Pat. No. (4,489,038) to Nylund. Nylund has a spacer comprised by upper and lower spaced frames interconnected by a resilient wall or strip. A pair of spaced apart stop projections embossed in the wall of the fuel assembly housing or channel constitute first locking means, and a stop member formed in the middle of the strip of the spacer constitutes a second locking means. When a fuel rod is inserted into the spacer cell adjacent to the stop member, it urges the resilient strip 5 toward the fuel assembly channel and thereby the stop member 6 from its inactive position to an active position where it is positioned so close to the channel that the spacer cannot be moved upwards or downwards without the stop member abutting against one of the projections 7 embossed in the channel. Preferably, the first and second locking means are located at each of the corners of the spacers and tubular channel.
However, none of these above-described arrangements appears to be the optimum solution to supporting spacers in a BWR fuel assembly so as to maintain their axially spaced positions. Consequently, the need exists for further improvement of the BWR fuel assembly with respect to spacer retention.